Organic thin-film solar cells can be produced by a non-vacuum process. Thus, they have the advantage that they can be mass-produced at low cost by a simple coating method such as printing, as compared with silicon solar cells, compound semiconductor solar cells, inorganic material-based solar cells such as CIGS type solar cells. If a flexible substrate made of resin, etc. is used, a high degree of design freedom can be achieved due to the flexibility thereof. Because of such characteristics, the organic thin-film solar cell is expected as a next-generation solar cell.
As an early organic thin-film solar cell, a pn-hetero-junction (flat hetero-junction) prototype cell composed of p-type and n-type organic semiconductors has been produced. However, the diffusion length of excitons is as short as around 10 nm. Thus, carriers are generated only in a region of several tens of nanometers from the interface of pn-junction and the conversion efficiency is extremely low. This problem has been resolved by a bulk hetero-junction (BHJ) technology of mixing the p-type organic semiconductor with the n-type organic semiconductor and spreading a pn-junction of nano-order to the whole thin-film. However, it is still difficult to produce an organic thin-film solar cell having sufficient conversion efficiency due to a narrow absorption spectrum width of a p-type organic semiconductor polymer to be used for a photoelectric conversion layer. In order to improve the conversion efficiency, there is a need for development of a new device structure and material.